Geochemical characterization of the Mahawelli River, Sri Lanka, based on basement rock, soil and sediment compositions

The geochemistry of river sediments is the product of the combination of many factors and natural processes, including source rock composition, climate, weathering, and sorting. At present few data are available for river sediments in Sri Lanka. To address this, we have examined the provenance, environmental, and geochemical characteristics of sediments from the 335km-long Mahawelli River, which cuts across the wet, intermediate, and dry zone climatic zones of Sri Lanka, in succession. The Mahawelli flows through the Highland Complex, which is characterized by coarse-grained granulite facies metamorphic rocks including charnockites, quartzites, marbles, garnet-gneisses, and granulites. River sediments (n=54) and selected soil (n=22) and basement rock samples (n=38) within the Mahawelli basin were analyzed for As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc, F, Br, I , Cl, Fe2O3, TiO2, MnO, CaO, P2O5 and total sulfur, using a Rigaku RIX-2000 XRF. The results show that Th is strongly correlated with V (0.86, p-value <0.01), Nb (0.86, p-value <0.01), Ti (0.89, p-value <0.01) and Zr (0.91, p-value <0.01). The distributions of these elements are thus likely controlled by heavy minerals including zircon, monazite, garnet, hornblende, amphibole, biotite, and titanite. Strontium is well correlated with CaO (0.97, p-value <0.01), suggesting the source of Sr was from carbonates in the basement. Average TiO2 contents of the river sediments (1.6 wt%) are greater than those of the soils (0.8 wt%) or basement rocks (0.6 wt%). The river sediments are also characterized by higher Fe2O3 (9.8 wt%) contents than the soils and basement rocks (7.7 and 5.7 wt% respectively). The higher TiO2 and Fe2O3 contents of the river sediments suggest intense weathering of the basement rocks, and accumulation of these elements in the soils and derived sediments. All of the elements analyzed except fluorine are enriched in the silt and clay fraction (<180 µm) of the river sediments relative to the sand fractions (180 - 2000 µm). The coarser fraction (180-2000 µm) forms the larger proportion in most of the river sediments, but there is no clear change in proportions of the fractions downstream. Immobile element ratios (Th/Sc, Zr/Sc, Ti/Zr) in the basement rocks show a range from intermediate to felsic compositions. Ratios in the stream sediments show a similar range, but Zr/Sc ratio and Th/Sc ratios are higher in some samples due to zircon concentration. Cr/V ratios of both basement rock and river sediment are low, indicating that ultrabasic sources are lacking, whereas Y/Ni ratios range from intermediate to felsic values typical of calc-alkaline rocks. The results of the elements As, Pb, Y, Cu, Sr, Zr, Th, Sc, F, Br, CaO, P2O5, Fe2O3 and TiO2 are concordant to the three divisions of the climatic zones. Spikes in the concentrations of elements Cr, V, Y, Nb, Zr, Th, Sc, TiO2 and Fe2O3 also occur below the confluences of tributary rivers. Key words: Mahawelli River, Sri Lanka, Sediments, Geochemistry, Provenance.